1. Field of the Invention
The present invention relates to a capacitance accelerometer, and more particularly, a capacitance z-axis accelerometer which can be integrated together with x- and y-axis accelerometers into a single chip and maximize the change of capacitance to achieve excellent acceleration sensitivity in the z-axial direction as well as utilize an amplifier and a filter of low cost.
2. Description of the Related Art
An accelerometer is known as a Micro Electro Mechanical System (MEMS) device. MEMS devices indicate microscale mechanical devices that are electrically controlled and measured, in which the MEMS is a technique for fabricating mechanical and electrical devices through the semiconductor process.
Various accelerometers capable of measuring acceleration are being currently developed, and adopted in vehicle air bag systems, Anti-lock Brake Systems (ABS) and general vibrometers. The accelerometers are mainly fabricated through the semiconductor process, and classified into piezoelectric, piezoresistant and capacitance accelerometers. Piezoelectric accelerometers are commercially retrogressing since it is difficult to prepare piezoelectric thin films of excellent properties without static characteristics. Further, piezoresistant accelerometers show a wide range of characteristic change according to temperature variation, which is hardly compensated. Therefore, the current technical trend is inclined to capacitance accelerometers.
The capacitance accelerometers have very excellent characteristics: A capacitance accelerometer shows a small level of characteristic change according to temperature variation, allows a field effect transistor of a high integrity to constitute a signal processing circuit without additional processes, and can be prepared at low cost.
FIG. 1 schematically illustrates a typical accelerometer. As shown in FIG. 1, a conventional capacitance accelerometer 1 includes a floatable mass 10 as a movable structure, suspension beams 22 and 24 functioning as springs of a mechanical stiffness for elastically supporting both ends of the mass 10, a plurality of movable electrode fingers 12 and 14 extended outward from the mass 10 into a bilaterally symmetrical configuration seen in the drawing, a plurality of fixed electrode fingers 32 and 34 fixed to both electrode-fixing sections 30a and 30b and spaced from the movable electrode fingers 12 and 14 to a predetermined gap and beam-fixing sections 20a and 20b for fixing the suspension beams 22 and 24 to the bottom of an insulation board. The movable electrode fingers 12 and 14 are adapted to maintain a fixed gap from the fixed electrode fingers 32 and 34 unless any acceleration is applied from the outside so as to keep a predetermined value of capacitance.
The reference numeral 19 designates an etching hole for introducing etching solution therethrough.
Upon application of an external force to the accelerometer 1, the mass 10 is displaced in the direction of the force or the y-axial direction (i.e., the vertical direction seen in the drawing), pulling the movable electrode fingers 12 and 14 fixed thereto in the y-axial direction. This as a result increases and decreases the gaps g1 and g2 from the movable electrode fingers 12 and 14 to the fixed electrode fingers 32 and 34, indicating the displacement of the mass 10.
This changes the capacitance between the movable electrode fingers 12 and 14 and the fixed electrode fingers 32 and 34. The change of capacitance is induced as current into the movable electrode fingers 12 and 14 according to a sensing voltage applied to the fixed electrode fingers 32 and 34, and the current is converted into a voltage and then amplified with an amplifier (not shown) connected to the movable electrode fingers 12 and 14 so that the external acceleration can be measured.
In order to measure external acceleration with respect to x-, y- and z-axes with the accelerometer 1, two accelerometers of this type are horizontally placed so that the mass 10 can be displaced in the x- and y-axial directions, and a third accelerometer of this type is vertically installed to measure the z-axial acceleration through the displacement of the mass 100 in the z-axial direction.
This structure has a problem that the total volume increases because the vertical z-axis accelerometer 1 increases the overall height occupying a large space together with the x- and y-axis accelerometers.
If the height of the z-axis accelerometer 1 is reduced to decrease the overall structure, the change of capacitance between the movable and fixed electrode fingers 12, 14, 32 and 34 becomes excessively small. Then, in order to detect and convert the trace amount of capacitance change into output voltage, it is necessary to provide an amplifier of high amplification rate, a demodulator and a high performance filter around the accelerometer 1. However, this also complicates the overall structure of the accelerometer and raises fabrication cost as well.
Furthermore, because noise components in an input signal are also amplified together, a higher amplification rate generates an output signal containing more noise and nonlinear components thereby further deteriorating the performance of the accelerometer.